Re: [RFC][PATCH 1/5] lib: add unpack_hex_byte()

[Andrew Morton <akpm@google.com>]

On Fri, 16 Sep 2011 08:50:26 -0400
Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:

> Since converting 2 ascii hex digits into a byte with error checks
> is commonly used, we can replace multiple hex_to_bin() calls with
> a single call to unpack_hex_byte().
> 
> Changelog:
> - Error checking added based on Tetsuo Handa's patch.
> - Moved the hex2bin code here, making it into a static inline function.
>   (Andy Shevchenko's request.)

Why, on earth?  Not for performance reasons - the code is already quite
inefficient.

> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
> ---
>  include/linux/kernel.h |   21 +++++++++++++++++++++
>  1 files changed, 21 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 46ac9a5..d8ea13b 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -385,6 +385,27 @@ extern int hex_to_bin(char ch);
>  extern void hex2bin(u8 *dst, const char *src, size_t count);
>  
>  /*
> + * unpack_hex_byte - convert 2 asii hex digits into a byte
> + * @byte: binary result
> + * @buf: ascii hexadecimal byte string
> + */
> +static inline bool unpack_hex_byte(u8 *byte, const char *buf)
> +{
> +	int hi, lo;
> +
> +	hi = hex_to_bin(buf[0]);
> +	if (hi < 0)
> +		return false;
> +
> +	lo = hex_to_bin(buf[1]);
> +	if (lo < 0)
> +		return false;
> +
> +	*byte = (hi << 4) | lo;
> +	return true;
> +}

Putting the return value into *byte is inefficient too - it forces the
compiler to place the return value into addressible storage (ie: into a
stack temporary).  Maybe the compiler can avoid doing that if the code
is inlined, but the code shouldn't be inlined :(

The return value is undocumented.

Why not copy the hex2bin return value semantics rather than creating a
new scheme?  Returning "bool false" in response to an error is very
unconventional and unexpected.  Kernel code returns a negative value on
error.

Wouldn't it be better to fix hex2bin() so that it returns -1 on error? 
Then the above function becomes a one-liner:

	return hex2bin(dst, src, 2);

Finally, the name is poor.  It starts with "unpack_", so it belongs to
the "unpack" subsystem.  There's no such thing.  Something like
hex_byte_to_bin() would be better.